Various wavelength conversion laser light sources have been developed and put into practice to obtain visible laser beams and ultraviolet laser beams. This kind of wavelength conversion laser light sources converts light emitted from Nd:YAG laser or Nd:YVO4 laser into visible green light, for example, on the basis of wavelength conversion employing the nonlinear optical effect. Alternatively, the wavelength conversion laser light source further converts the converted green light into ultraviolet light. The aforementioned visible and ultraviolet lasers are used, for example, in laser process for materials or as a light source of a laser display device.
This kind of laser light sources includes a solid-state laser medium such as Nd:YVO4 and a wavelength conversion element such as lithium niobate. Patent Documents 1 to 3 propose a microchip type wavelength conversion laser light source (optical contact-type wavelength conversion laser light source) comprising a solid-state laser medium and a wavelength conversion element, which are optically bonded without adhesive. A microchip type wavelength conversion laser light source outputs approximately 10 mW at maximum. The microchip type wavelength conversion laser light source is used, for example, as a light source of a laser pointer and a collimator.
The microchip type wavelength conversion laser light source of Patent Document 1 aims to improve production yield in optical bonding processes between the solid-state laser medium and the wavelength conversion element. To achieve the objective, Patent Document 1 proposes enlargement of the bonding area between the solid-state laser medium and the wavelength conversion element.
The microchip type wavelength conversion laser light source of Patent Document 2 comprises an optical thin film disposed at the interface between the solid-state laser medium and the wavelength conversion element. Patent Document 2 proposes a method for bonding the solid-state laser medium with the wavelength conversion element to inhibit light scattering. According to the prior art documents, it is necessary to reduce, as much as possible, scattering and losses of light which propagates through the interface (bonding surface) between the solid-state laser medium and the wavelength conversion element.
If a laser beam is incident for long hours to an optical element, which is formed by combining the solid-state laser medium with the wavelength conversion element, output of the laser beam from the optical element goes down over time.
FIG. 50 is a plot diagram showing problems on the output declination of the aforementioned laser beam. The output declination of the laser beam is described with reference to FIG. 50.
The vertical axis of the plot diagram of FIG. 50 shows laser output intensity (harmonic output) from the optical element. The horizontal axis of the plot diagram of FIG. 50 shows driving time of the optical element. FIG. 50 shows characteristics of an optical element comprising a solid-state laser element and a wavelength conversion element in optical contact. The optical element used for obtaining the plot diagram of FIG. 50 converts a wavelength of an externally input excitation laser beam to output a green laser beam.
According to the plot diagram of FIG. 50, the output declination of the green laser occurs several ten hours after the start of green laser beam emission from the optical element. It is less likely that the output declination of the emitted light is observed if the output of the laser beam subjected to the wavelength conversion is approximately 100 mW. If the optical element emits light with a peak output of 500 mW or more (in particular 1000 mW or more), it is confirmed that there is considerable output declination.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-102228    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2008-16833    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-357834